To date, suppression of aircraft turbo-fan engine vibration has utilized hydraulically-powered oscillating-mass linear force generators mounted orthogonally on the engine strut bulkhead which carries the aft engine mount. While this technique has proven effective and has the potential for reducing forces at multiple frequencies, problems have developed with respect to waveform fidelity (e.g., the generation of unwanted harmonics). Moreover, this form uses significant hydraulic power.
The use of geared counter-rotating eccentric masses to generate an oscillatory force has been known. This type of device has been applied to apparatus such as conveyer shakers and vibration testers. Further, adjustable differential gearing has been used to vary the phase relationship of two summed oscillatory forces to control the effective amplitude of the resultant force. U.S. Pat. No. 5,005,439 shows a logical extension of this type of device, combined with state-of-the-art motor control techniques, to provide four independently-controlled concentric co-planar nested rotating masses capable of generating a variable-amplitude adjustable-angle oscillatory force at a single point. A rotating eccentric mass force generator has the fundamental advantages of producing a pure sinusoidal source (albeit at a single frequency), and requires only enough power to overcome bearing friction, thus making electric motor drive practical. However, the device as shown in said U.S. Pat. No. 5,005,439 appears to be mechanically complex and awkward to package. While the generated force is equivalent to a pair of orthogonal hydraulic linear force generators acting at a single point, it is believed that the desired counter-vibration pattern required to cancel engine vibrations is necessarily more complex.